1. Field of the Invention
This invention relates to a process for the production of monounsaturated fatty acids or derivatives thereof by catalytic hydrogenation of polyunsaturated fatty acids or derivatives thereof.
2. Statement of Related Art
Technical fatty acids or fatty acid derivatives having high oleic acid contents, for example of more than 60% or more than 70%, are starting materials in great demand for products of the cosmetic industry, the oil production industry, the textile industry and for many other applications.
The industrial processes hitherto used for the partial hydrogenation of fatty acids and their derivatives lack selectivity and are difficult to carry out. Hitherto, the processes in question have mostly been carried out with heterogeneous nickel catalysts at high temperatures in the range from 120.degree. to 180.degree. C. In processes involving the throughflow of hydrogen, pressures of up to 3 bar are applied. In so-called "dead-end" processes, pressures in the range from 1 to 3 MPa (10 to 30 bar) are applied. Unfortunately, these processes lack selectivity in regard to the oleic acids formed and, instead, give technical fatty acid cuts with considerable amounts of saturated fatty acids, particularly stearic acid, which are troublesome in many industrial applications and have to be removed from the oleic acid cuts at considerable expense.
The hydrogenation of linoleic acid methyl ester with nickel catalysts is described in Izv. Akad. Nauk SSSR, Ser. Khim. 3, 642 (1979). Before the reaction, the catalyst has to be subjected to a complicated activation process lasting several hours. This process cannot be carried out on an industrial scale on account of the unfavorable educt-to-catalyst ratio of 25:1. In addition, the hydrogenation is not selective; in nickel/ methyl formamide systems, 12% methyl stearate is formed as secondary product; in nickel/dimethyl acetamide systems, 15% methyl stearate is formed as secondary product.
Finally, it is known from J. Cat., 24,536 (1972) that fatty acid derivatives can be hydrogenated in the presence of complexes of a transition metal and an aprotic amidic solvent, for example dimethyl formamide. However, the rhodium and nickel catalysts described in this reference also have to be subjected before the reaction to a very complicated pretreatment with equimolar quantities of the expensive reducing agent, sodium boranate. However, the process in question cannot be carried out on an industrial scale because it requires extremely large quantities of catalyst. Where rhodium is used as the catalyst, a maximum educt-to-catalyst ratio of 10:1 is possible whereas, where nickel is used as catalyst, a corresponding ratio of only 5:1 is possible. In addition, in the production of unsaturated fatty acids for applications in the cosmetics industry and in the food industry, there is the disadvantage that the use of amide-containing complexing agents is undesirable on ecotoxicological grounds.
Accordingly, the problem addressed by the present invention was to provide a process for the production of monounsaturated fatty acids or derivatives thereof which would be free from the disadvantages described above.